1. Field of the Invention
The present invention relates to a variable valve timing mechanism that varies the timing of the opening and closing the valves in a four-cycle internal combustion engine.
2. Description of Background Art
A variable valve timing mechanism has the following structure. An eccentric shaft that rotates independently from the rotation of a camshaft is disposed inside the camshaft. An eccentric collar having two sandwiching portions located on two sides across the center thereof is supported by the outer circumference of an eccentric portion of the eccentric shaft with rollers, as clearance-securing members, interposed in between. The eccentric collar thus supported is capable of rotating eccentrically. A driving collar having a driving projection that engages with a first one of the sandwiching portions of the eccentric collar is attached to the outer circumference of the camshaft and thus is integrated with the camshaft. A valve-lifting cam member having a driven projection that engages with a second one of the sandwiching portions of the eccentric collar is attached onto the outer circumference of the camshaft. The valve-lifting cam member thus attached is capable of sliding in the circumferential direction. The torque of the driving projection of the driving collar that rotates integrally with the camshaft is transmitted to the driven projection of the valve-lifting cam member via the pair of the sandwiching portions of the eccentric collar. The valve-lifting cam member is driven while the rotational phase thereof is cyclically varied. The eccentric shaft is provided to adjust and set the center position of the eccentric collar by use of the eccentric portion. See, for example, Japanese Unexamined Patent Application Laid-open Publication No. Sho63-1707 (FIG. 4 and FIG. 6)).
In a variable valve timing mechanism a pin is conventionally used to fix a driving collar onto a camshaft so as to rotate integrally with the camshaft. In the variable valve timing mechanism in which the pin is used, the pin has to be pressed to fit, so that the assembling operation of the driving collar to the camshaft and the maintenance operation that needs the detaching of the pin become complicated. In addition, a space for a pin hole has to be secured, so that the cylindrical portion of the driving collar becomes larger in size in the axial direction. As a consequence, the variable valve timing mechanism becomes larger in size. Moreover, forming the hole for this purpose needs a larger driving collar so as to secure a sufficient strength of the driving collar.
A conventional structure of eccentric shaft includes a shaft portion and an eccentric portion. The eccentric portion has a smaller diameter than that of the shaft portion, and has its center that is offset from the center of the shaft portion. When a large force is applied on a first one of the control driving portion of the eccentric shaft and the valve-lifting cam member, the large force is transmitted to a second one of the two members. The two members, thus, have to be made stronger. This leads to an increase in weight of the eccentric shaft.
In a conventional variable valve timing mechanism, the eccentric collar eccentrically rotates in a state where a plurality of rollers are disposed to secure a certain clearance between the eccentric collar and the outer circumference of the eccentric portion. The rollers are held respectively in the retaining windows formed in the camshaft. When the internal combustion engine runs normally, the eccentric portion of the eccentric shaft is stopped, the camshaft rotates, and the eccentric collar eccentrically rotates along with the rotation of the camshaft. Accordingly, the relative amount of rotation of the camshaft to the eccentric collar is small, while the relative amount of rotation of the camshaft to the eccentric portion of the eccentric shaft is large. Accordingly, each of the rollers does not actually roll, but slides on a particular portion thereof that is in contact with the eccentric portion of the eccentric shaft. In this particular portion, the convex surface of the roller is in contact with the convex surface of the eccentric portion, so that the surface pressure is high. As a consequence, securing the durability of these rollers is difficult.